Molten metals usually contain solids such as oxides of the metal and other impurities which may cause the final cast product to have undesirable characteristics. Filters have been devised to remove these impurities from the molten metal during the casting process. Normally these filters are made of refractory materials to withstand the high temperatures associated with molten metals.
One type of filter is a cellular filter which comprises a series of parallel ducts or passages for metal to pass through. Such filters are formed by extrusion or by stamping. Although they are robust and easy to handle, their filtration efficiency is relatively poor because the molten metal only travels a short and straight path through the filter.
The preferred refractory filters have a foam-like appearance and are referred to in the metal filtration industry as foam filters. These are usually ceramic foam filters but more recently carbon-bonded filters (where the refractory material is bonded by a material comprising a carbon matrix, as described in WO2002/018075) have started to become established for certain applications. A foam filter has a network of strands which define a plurality of interconnected open cells. Since the flowpath through such a filter is tortuous, the filtration efficiency is much higher than that of the cellular filters.
The fabrication of ceramic foam filters is described in EP 0 412 673 A2 and EP 0 649 334 A1. Typically; an open celled foam (e.g. reticulated polyurethane foam) is impregnated with an aqueous slurry of refractory particles and binder. The impregnated foam is compressed to expel excess slurry and then dried and fired to burn out the organic foam and to sinter the refractory particles and binder in the slurry coating. A solid ceramic foam is thereby formed having a plurality of interconnecting voids having substantially the same structural configuration as the starting foam. Although the filtration efficiency is much improved over the previously described cellular filters, ceramic foam filters are mechanically weaker (the strands, particularly at the edge of the filter are prone to breakage).
In use the filter may be placed in an opening in a wall between a molten metal inlet and a molten metal outlet to filter the metal. One example of the placement of a filter in a refractory wall is described in U.S. Pat. No. 4,940,489. Since the foam filters are porous in all directions and the edge surfaces are uneven, it is possible for some molten metal to flow around the edges of the filter or only pass through part of the filter, thereby reducing filtration efficiency. This problem is exacerbated if strand breakage has occurred during transport of the filter or during positioning of the filter in the refractory wall (it will be noted that the broken strands themselves can contribute to the impurities in the final casting).
Increasing the amount of slurry used to impregnate, i.e. coat the foam, in the production of the filter increases its strength but also results in reduced filtration efficiency due to the higher weight and reduced porosity.
The process of filtration requires the filter to be primed, in which the filter pores are filled with metal and a continuous flow of metal is achieved. Priming involves the displacement of air in the pores (at the surface of the filter) and the pressure required is inversely proportional to the size of the pores. In addition, temperature losses in the metal will increase metal viscosity hence filters with a high heat capacity will cause increased thermal losses and reduce priming. A heavier filter in which the coated strands are thicker is therefore undesirable since it will have a greater heat capacity. This means that the molten metal will need to be heated to a higher temperature to ensure that it does not freeze as it passes through the filter. This is disadvantageous from both an economic and environmental standpoint as it increases the amount of energy required to heat the metal to the required temperature.
In addition to the extra weight, filters produced using an increased amount of slurry will have reduced metal flow rates due to the increased strand thickness and smaller pores, and will have a greater tendency to block. Reduced flow rates and premature blockage can have adverse effects on metal casting, for example, by increasing pouring times or causing incomplete mould filling, and it may be necessary to increase the size of the filter or to increase the pore size of the foam. Increasing the level of slurry is therefore not a practical solution to increasing the strength of foam filters, particularly the edges of foam filters.
U.S. Pat. No. 5,039,340 describes a method for the manufacture of a foam filter where an adhesion promoting material, preferably together with flocking, is applied to the foam. The adhesion promoting material and flocking increase the amount of slurry that subsequently adheres to the foam. The end result is a stronger but heavier filter.
It has previously been proposed to provide the edges of the foam filter, which contact the wall of the mould/die, with a protective layer. The purposes of this protective layer can include enhancing mechanical strength, preventing the passage of metal between the mould or die wall and the filter (metal by-pass), and reducing the likelihood that the ends of the ceramic foam filter strands will break off during handling (particularly mechanical/robotic handling of filters) and transport. The protective layer also facilitates the use of robotic handling to allow automatic placement of the filters in moulds.
EP 0 510 582 A1 discloses a ceramic foam filter encased in a rigid frame of metal or ceramic. The ceramic framed filter can be made by wrapping an extruded strip of dough-like ceramic forming mass around the filter, which may or may not have been pre-fired, and then drying and firing.
CN 200991617Y discloses a ceramic foam filter having a protective layer of organic material around its edge which decomposes at high temperature during the use of the filter. The protective layer is said to reduce damage to the filter during transport and installation and also allow for its use in automatic production lines.
U.S. Pat. No. 4,568,595 relates to a ceramic foam filter having a ceramic coating. The coating is provided by trowelling, brushing or spraying a ceramic slurry over the fired ceramic foam filter and then firing the composite structure.
U.S. Pat. No. 4,331,621 describes a ceramic foam filter having an integrally bonded ceramic gasket secured to a peripheral surface thereof. It can be made by impregnating a flexible foam material with a slurry, placing it in a mould having the desired size of the final filter product and then feeding a ceramic fibre slurry into the gap between the foam material and the mould. The mould is then dried and fired to burn out the foam and sinter the ceramic material.
GB 2 227 185 suggests, in one embodiment, saturating a foam plastic starting piece with ceramic slip and then squeezing the foam to urge the surplus slip into a solid peripheral layer before firing. In another embodiment GB 2 227 185 proposes forming a closed layer on a ceramic foam filter by adhering either a further foam material or a web of fine plastic filaments to the foam. During impregnation with the slip, the small pores or intermediate spaces in the peripheral side edge region become, and remain, filled with slip, thereby forming the closed layer on firing. In both embodiments, the resulting coating is thick thereby reducing the useful volume of the filter and also increasing its heat capacity.